The present invention relates to semiconductor materials and, more particularly, to polishing the surface of a semiconductor wafer of a silicon or other material from which solid state electronic devices are fabricated.
The basic substrate material for the solid state electronics industry is the silicon wafer, i.e., a flat disc of single crystalline silicon having a thickness of about 0.015 inches (0.38 mm). The wafers are generally produced by first growing an elongated single crystal of the silicon or other semiconductor material, and then slicing the same into wafer form. One side surface of each wafer, which side surface is referred to herein as the front or front side surface, is then highly polished on apparatus designed specifically for such purpose. Such surface is required to be polished in order to form reliable semiconductor junctions with other materials which are applied in thin films thereto.
Generally, the polishing apparatus includes a backing plate or carrier to which unpolished wafers are adhered, with the wafer surfaces to be polished exposed to a polishing pad which engages the same with polishing pressure. The polishing pad and carrier are then typically both rotated at differential velocities to cause relative lateral motion between the polishing pad and such wafer front side surfaces. A colloidal silica slurry is generally provided at the polishing pad-wafer surface interface during the polishing operation to aid in the polishing.
The wafers are generally adhered to the carrier for the polishing operation by a thin wax or resin film. However, the quality level of finished wafer products required for some applications has increased to the point that wax adherence is no longer satisfactory in many instances. In this connection, when a wax or similar film is used to mount the wafers, any random variations in film thickness, dust particles or other contaminations in the wax film, or any asperities on the back surface of the wafer, i.e., that surface of the wafer which is adhered to the carrier, will cause the polishing operation to form irregularities on the wafer front surface. This results in the finished polished surface not having the required flatness. Moreover, the wax film is, in effect, a contaminant which must be removed (often in a relatively complicated manner) from the wafer after the polishing operation.
In order to avoid the disadvantages inherent in wax mounting, workers in the field have devised other, wax-free, methods of securing a wafer to a backing plate for the polishing operation. For example, templates made of MYLAR.RTM. or a similar material having apertures which accept the wafers have been mounted on the carrier. Wafers are typically adhered within the template apertures by the surface tension of a water or the like film. A major shortcoming of this technique is that the wafers tend to slide around within the apertures (particularly at high polishing pressures), increasing the likelihood of wafer breakage and intermittently polishing the wafer back surface. Another approach that has been proposed is the use of vacuum suction to adhere the wafers to the backing plate. Difficulty has been experienced with this approach, however, in attempts to prevent the polishing slurry from being sucked beneath the wafers in those areas in which they only partially contact the carrier. Also, such an arrangement is extremely sensitive to the presence of dust particles or of discontinuities in the backing plate surface.
It has also been proposed that a frictional material, such as silicone rubber, be used to adhere the wafers to the carrier. While such a material will provide a high resistance to lateral wafer sliding, it typically will not support a wafer uniformly over its surface area and the polishing removal rates are correspondingly non-uniform across the wafer surface. This will cause the finished polished surface to be uneven and display waves (variations in polished surface flatness over significant distances) or crowning.